auto-fill-mode and filling paragraphs differ

[David Kastrup]

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Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

The setting of fill-column is interpreted differently for text entry
in auto-fill-mode and when manually calling fill-paragraph.  That is
a nuisance.

Take the following line in text-mode, auto-fill set and fill-column
at its standard value of 70.

dfsa fsda fds fs af sdaf sdaf sda fsa fsd afds af dsaf sda gfsda fsafd
sad fdsafds

Now if get to the end of the first line and press <SPC>, the final
fsafd will get wrapped to the next line.  If I then press M-q, it
will get wrapped back again.

The documentation of fill-column states:

    *Column beyond which automatic line-wrapping should happen.

I would interpret this as the last permissable column of content.
According to C-x =, column 70 is the column _after_ fsafd.  If we now
say "ok, at this column, the end of line is legal", it would appear
that auto-fill-mode wraps one column too early.  I think that typing a
space immediately at the end of line should _not_ wrap the preceding
word to the next line, but merely replace the typed space (instead of
the last space before that) with a newline.  Namely: when looking for
the last feasible breakpoint in the line, allow the just typed space
to qualify, too, if it starts right at the fill-column.

In GNU Emacs 21.3.50.16 (i686-pc-linux-gnu, GTK+ Version 2.2.1)
 of 2004-02-29 on lola.goethe.zz
configured using `configure '--prefix=/usr/local/emacs-21' '--with-gtk''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Major mode: Text

Minor modes in effect:
  file-name-shadow-mode: t
  auto-compression-mode: t
  desktop-save-mode: t
  display-time-mode: t
  iswitchb-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  unify-8859-on-encoding-mode: t
  line-number-mode: t

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum